build: fix errors spotted by ineffassign linter

These were mostly caused by shadowing err and a good fraction of them
will have caused errors not to be propagated properly.

backend/cache/cache.go

@@ -647,7 +647,6 @@ func (f *Fs) NewObject(remote string) (fs.Object, error) {
 
 	// search for entry in source or temp fs
 	var obj fs.Object
-	err = nil
 	if f.tempWritePath != "" {
 		obj, err = f.tempFs.NewObject(remote)
 		// not found in temp fs

backend/cache/cache_internal_test.go

@@ -988,8 +988,9 @@ func TestInternalUploadTempFileOperations(t *testing.T) {
 		require.Error(t, err)
 		_, err = os.Stat(path.Join(runInstance.tmpUploadDir, id, runInstance.encryptRemoteIfNeeded(t, "second/one")))
 		require.NoError(t, err)
-		started, err := boltDb.SearchPendingUpload(runInstance.encryptRemoteIfNeeded(t, path.Join(id, "test/one")))
+		_, err = boltDb.SearchPendingUpload(runInstance.encryptRemoteIfNeeded(t, path.Join(id, "test/one")))
 		require.Error(t, err)
+		var started bool
 		started, err = boltDb.SearchPendingUpload(runInstance.encryptRemoteIfNeeded(t, path.Join(id, "second/one")))
 		require.NoError(t, err)
 		require.False(t, started)

backend/cache/handle.go

@@ -446,7 +446,6 @@ func (w *worker) run() {
 					continue
 				}
 			}
-			err = nil
 		} else {
 			if w.r.storage().HasChunk(w.r.cachedObject, chunkStart) {
 				continue
@@ -493,7 +492,7 @@ func (w *worker) download(chunkStart, chunkEnd int64, retry int) {
 	}
 
 	data = make([]byte, chunkEnd-chunkStart)
-	sourceRead := 0
+	var sourceRead int
 	sourceRead, err = io.ReadFull(w.rc, data)
 	if err != nil && err != io.EOF && err != io.ErrUnexpectedEOF {
 		fs.Errorf(w, "failed to read chunk %v: %v", chunkStart, err)